1. Field of the Invention
This invention relates to an insulated gate bipolar transistor, and in particular, to a double gate insulated gate bipolar transistor (DG IGBT), and a method of manufacturing thereof.
2. Description of the Prior Art
An insulated gate bipolar transistor (IGBT) features a MOS gate for high impedance voltage-control and bipolar conduction for reducing the drift on-state resistance (through ‘conductivity modulation’). It can be seen as a successful combination between a power MOSFET and a bipolar transistor.
Various versions of IGBTs have been developed. One is a trench IGBT (FIG. 1), which is a variant of an IGBT featuring a plurality of vertical channels 50a. Referring to FIG. 1, the trench IGBT device features an n− drift region where the voltage is supported during the blocking mode in the off-state. The doping and depth of this region are given by the breakdown rating. A p well is placed on top of the drift region and is shorted to the cathode via a p+ region. The insulated gate is made of a thin oxide layer and a polysilicon layer, where both are placed inside a trench. This insulated gate is acting as the MOSFET structure gate. The n+ cathode layer also acts as the source for the MOSFET and the n− drift region becomes the drain. Upon application of a positive voltage between the gate and the cathode, which is greater than the threshold voltage, a channel 50a is formed inside the p-well at the interface with the gate oxide, which allows transport of the electrons from the n+ cathode into the drift region. This drift region is also the base of a pnp transistor featuring the p_ anode as its emitter, the n-drift region as its base, and the p well as its collector. During the on-state, high carrier injection of holes from the anode layer (i.e. emitter of the pnp transistor) and electron injection from the channel and the accumulation layer formed around the gate at the surface of the n-drift region yields conductivity modulation of the drift region; and as a result, a substantial decrease in the on-state resistance is provided. As in the Trench MOSFET, the n+ sources are self-aligned to the trench or displaced around the trench; and the overall dimensions of the cell can be made much smaller than in conventional IGBTs. This means that the channel density Z/A (where A is the active area of the device and Z is the perimeter of the channel 50a) is considerably larger than that found in the conventional IGBT. This yields a smaller channel resistance; and as a result, a smaller on-state voltage drops is present in the Trench IGBT when two devices are operating at the same current densities. The trench IGBT structure has also a more one dimensional natural flow, thereby avoiding bends and removing the parasitic JFET effect.
Referring to FIG. 2, a simple equivalent circuit model for the trench IGBT as shown in FIG. 1 or indeed any common IGBTs is that of an n-channel MOSFET driving the wide base of a pnp bipolar transistor. It depicts the pnp transistor whose base is connected to the MOSFET structure. This MOSFET is an enhancement mode MOSFET. The small p base resistor models the flow of the hole current in the p well. Under heavy modulation during the on-state, the base of the pnp transistor, which is called the drift region of the device, becomes highly conductive, thus minimizing the voltage drop across it. The n-channel MOSFET is active on positive signals being applied to the gate, and can turn into a p-channel when negative voltages are being applied.
Recently, many different types of IGBT have been developed, such as, for example, the following: 1) Punch-Through IGBT (PT IGBT) based on an epitaxial drift region grown on a highly doped substrate; 2) “Non Punch Through” IGBT (NPT-IGBT) based on homogeneous substrate material (float zone) having increased robustness and improved plasma distribution which overall cuts the switching losses in spite of its increased drift length; 3) Soft Punch Through IGBT (SPT-IGBT), also known as Field Stop IGBT (FS-IGBT) based on having a punch-through type drift region (similarly to that of the PT IGBT) but having a lightly doped buffer and a lightly doped and transparent anode instead.
Several recently reported IGBT structures which aim to improve the conductivity modulation at the cathode side of the drift region are as follows: 4) Injection Enhanced Gate Transistor (IEGT), which is a Trench IGBT with a lower n+ and p+ cathode contact area; 5) Carrier Stored Trench Bipolar Transistor (CSTBT) is a device featuring a ‘real’ PIN diode as it has an ‘n’ layer placed under the p-well; 6) High Conductivity IGBT (HiGT) featuring an n layer around the p-well to stop the holes reaching the cathode shorts and thus enhancing the electron injection at the top side of the drift region; 7) and Double Gate IGBT (DG IGBT), which includes devices having an extra cathode gate for enhanced modulation.
The IGBT is controlled via an n-channel MOSFET gate for turn-on and turn-off. To speed up the device, it is known that a p-channel MOSFET can be formed by applying a negative voltage to the gate of the MOSFET. This p-channel gate can be physically the same gate as the n-channel gate, but is operated with the opposite polarity or indeed can physically be a different gate, in which case, the device can be regarded as a double gate IGBT (DG IGBT).
In addition to the more detailed description of many of the various types of IGBT species above, one should also possess an understanding of the more generic form of transistors of which the IGBTs belong, namely, a MOSFET. The following descriptions refers now to a MOSFET, as this is a key component structure of the IGBT and plays an important role in the description of the invention.
The MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) is a semiconductor structure comprising two regions of a specified conductivity, known as a source region and a drain region, and are separated from each other by a another region of opposite conductivity, known as a base region or base layer. Charge carriers of the specified conductivity type can flow between the source and drain through a gate-controllable channel of the specified conductivity type extending across a surface of the opposite conductivity base layer. The gate is an electrically conductive electrode, commonly formed of polysilicon material situated over an electrically insulating layer of oxide over the semiconductor surface which contains the channel. Conductivity of the channel, and hence the current flowing between source and drain regions, is changed as changes are made to the voltage at which the gate is biased relative to the base region/layer. The device is known as an n-channel or p-channel MOSFET in accordance with the conductivity type of the source, drain, and the channel regions. The device is known as an enhancement-mode or depletion-mode MOSFET in accordance to whether an applied gate bias produces an enhancement or depletion of the conductivity of the channel.
An enhancement-mode structure is normally off, and does not conduct current unless a bias voltage is applied to the gate with respect to the source.
A depletion-mode MOSFET structure contains a pre-formed channel that is normally on, and will conduct current when no bias voltage or zero bias is applied to the gate, but this current flow can be interrupted by an appropriate bias voltage applied to the gate with respect to the source.
Current flowing in a semiconductor can consist of negative charge carriers (electrons), or positive charge carriers (holes), or both. Unipolar conduction consists of only one species of charge carrier, and bipolar conduction consists of both species. Conduction in an N channel MOSFET is unipolar, since it consists of the flow of only electrons. Similarly, conduction in the p channel MOSFET is unipolar, since it consists of the flow of only holes.
As already mentioned, an IGBT incorporates an enhancement mode n-channel MOSFET to drive the base of a pnp bipolar transistor. As known in state of the art, a double gate IGBT can additionally incorporate an enhancement mode p-channel gate, that helps to speed up the hole removal upon applying a negative voltage between the gate and the source/cathode.
FIG. 3 illustrates a conventional double gate device as described in U.S. Pat. No. 5,554,862. The aforementioned conventional double gate device is an IGBT device comprising a p+ emitter layer 1a, a high-resistance n− base layer 3a, a second p base layer 4b, an n buffer layer 2a, a p emitter layer 1a, a trench structure 20a, a first p base layer 4a, an insulating film 22a, a trench 21a, a polysilicon layer 23a, a first electrode 10a, a gate insulating film 9a, a p+ source layer 12a, a second gate electrode 11c, a cathode electrode 8a, an anode electrode 7a, a cathode emitter electrode 19a, a high-concentration p+ layer 27a, and a vertical groove portion 66a. 
The aforementioned conventional double gate device in FIG. 3 requires a second, a p-channel, and a gate operating only when a negative voltage with respect to the cathode terminal, K is applied to it. As a result, there is a need for having complex and expensive drive circuits for providing the negative voltages to the second (p-channel) gate 11c. 
An equivalent circuit model of the device shown in FIG. 1 is schematically shown in FIG. 4. Referring to FIG. 4, a p-channel MOSFET is active on negative signals when applied to its gate. Meanwhile, an n-channel MOSFET is active on positive signals when applied to its gate. It is also an equivalent circuit model of the device shown in FIG. 3 when the two insulated gates are separate (G1 and G2). In addition to the equivalent circuit model as shown in FIG. 2, the equivalent circuit model in FIG. 4 features a second gate. This second gate is part of an enhancement mode p-channel transistor that is active when a negative voltage is applied to the second gate with respect to the cathode. This is of help to speed up the extraction of excess charge from inside the drift region and thus reduce the turn-off time of the device. However, the presence of both positive and negative gate voltages (e.g. +15 V and −15 V) results in a significant increase in the complexity and cost of the drive system for this device (not shown).
For other IGBTs using the enhanced injection concept, large p resistors placed along large surface tracks in the third dimension (as described in U.S. Pat. No. 7,170,106 and U.S. Pat. No. 6,809,349) tend to occupy considerably more space than the pnp transistor, as well as creating distributed paths for the holes which can lead to the non-uniform modulation of the drift region.
In short, the conventional IGBT devices at a particular set of anode injection and breakdown ability conditions do not provide adequate on-state and turn-off characteristics, and may suffer from higher on-state saturation current.